(a) Field of the Invention
The present invention relates to a copper foil to be used for the production of TAB (Tape Automated Bonding) tape carriers and a TAB carrier tape and a TAB tape carrier both using the copper foil. Particularly, it relates to a copper foil for TAB tape carriers, which effectively prevents the generation of Sn whiskers and Kirkendall voids, and a TAB carrier tape and a TAB tape carrier using the copper foil.
(b) Description of the Related Art
To automate and speed up the mounting of semiconductor elements, there have been extensively used the TAB technique wherein a semiconductor element, such as IC or LSI, is mounted by wireless-bonding on a tape carrier, which is a lengthy, flexible insulating film bearing copper lead patterns.
Being excellent in corrosion resistance and soldering capability, Sn-plating film has recently been formed on copper lead patterns in substitution for Au-plating film. The Sn-plating film, however, is known to generate with passage of time Sn whiskers, which are beardlike, needle-like crystals. The growth of Sn whiskers causes short circuit. To prevent this, after Sn-plating, heating treatment for 1 to 2 hours at 100 to 150xc2x0 C. has been required. The heating, however, causes rapid counter diffusion between the Sn-plating film and Cu of copper foil, and, because Cu atoms diffuse faster than Sn atoms, voids, what are called Kirkendall voids, are made on the side of the copper foil contacting the Sn-plating film. This lowers the adhesion of the Sn-plating film, causing peeling on soldering.
As the pitch of copper lead patterns has recently been made finer, prevention of the generation of Sn whiskers and Kirkendall voids has become an important object to be achieved promptly.
There have been taken various measures to prevent the generation of Sn whiskers and Kirkendall voids. For example, a means proposed to prevent the generation of Sn whiskers is the use of a tin plating bath to which indium and a salt of a low-melting metal, such as lead, bismuth or antimony, are added in such an amount as to form a deposit film containing 0.1 to 3.0% by weight of the low-melting metal. It is also proposed to plate as a backing for the tin-plating film a metal other than tin, such as backing solder-plating or backing nickel-copper two-layer plating.
To prevent the generation of Kirkendall voids, it is proposed to inhibit the counter diffusion of Cu and Sn by forming a Zn layer or a copper alloy layer containing at least 10 % by weight of Zn at the parts for bonding with solder.
These methods, however, involve the problems that (1) a step is added prior to Sn-plating, thereby increasing the production cost and lowering productivity and (2) copper foil itself cannot be improved.
An object of the present invention is to provide a highly reliable copper foil for TAB tape carriers, which effectively prevents the generation of Sn whiskers and Kirkendall voids without requiring any addition or change of steps after the formation of copper lead patterns, and to provide a TAB carrier tape and a TAB tape carrier both using the copper foil.
Another object of the present invention is to provide a copper foil for TAB tape carrier, which maintains good properties necessary for forming copper leads, including heat resistance and solder wettability.
As the result of study for achieving the objects, we have found that the generation of Sn whiskers on the Sn plating film formed on patterned copper leads and the generation of Kirkendall voids due to the counter diffusion of Cu and Sn can be outstandingly prevented by using as a copper foil for TAB tape carriers a copper foil having on its shiny surface (S-surface) an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer), and have made the present invention based on the finding.
The present invention provides a copper foil for TAB tape carriers, comprising
(a) a copper foil having a shiny surface and a mat surface; and
(b) an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer), which is formed at least on the shiny surface.
The present invention further provides a TAB carrier tape, which comprises a flexible insulating film and the above-described copper foil for TAB tape carriers applied onto the flexible insulating film, with a surface on the side of the mat surface contacting the flexible insulating film.
The present invention further provides a TAB tape carrier, which is produced from the above-described TAB carrier tape by etching the copper foil for TAB tape carriers to form a copper lead pattern.
The copper foil for TAB tape carriers of this invention is characterized in having on at least the shiny surface (S-surface) of a copper foil an alloy layer comprising nickel, cobalt and molybdenum (nickel-cobalt-molybdenum layer). The nickel-cobalt-molybdenum layer formed on the copper foil is a thin alloy layer comprising metallic nickel, cobalt and molybdenum as main components, and is firmly adhering to the surface of the underlying copper foil. Near the surface of the nickel-cobalt-molybdenum layer, nickel and oxides and hydroxides of cobalt and molybdenum may be mixed in. The copper foil having the nickel-cobalt-molybdenum layer generates less Sn whiskers and less Kirkendall voids after Sn-plating following to the formation of copper lead patterns, thereby preventing short circuit and solder peeling, and also maintains good properties necessary for forming the copper leads, including etching capability, heat resistance and solder wettability.
As to the coating amounts and ratios of nickel, cobalt and molybdenum constructing the nickel-cobalt-molybdenum layer as measured as metals, the coating amount of each of nickel, cobalt and molybdenum is preferably 30 to 1,000 xcexcg/dm2. If they are less than 30 xcexcg/dm2 respectively, the layer may be too thin to prevent the generation of Sn whiskers and Kirkendall voids sufficiently, and if more than 1,000 xcexcg/dm2, the generation of Sn whiskers and Kirkendall voids will be prevented sufficiently, but the etching capability for forming copper lead patterns may be adversely affected depending on the ratios among them. Each coating amount is particularly preferably 50 to 500 xcexcg/dm2.
In the present invention, electrolytic copper foil having a shiny surface (S-surface) and a mat surface (M-surface) is generally used as the starting copper foil. The copper foil may have any thickness of copper foil commonly used for TAB tape carriers, preferably 3 to 70 xcexcm, more preferably 5 to 18 xcexcm.
The surface roughness Ra of the S-surface of the copper foil is preferably 0.1 to 0.5 xcexcm, and that of the M-surface is preferably 0.2 to 0.7 xcexcm. Of course, the M-surface is rougher than the S-surface. To form fine copper lead patterns, the copper foil used in the present invention preferably has an M-surface with a surface roughness smaller than that of common copper foil. Further preferred surface roughness Ra of the M-surface is 0.2 to 0.4 xcexcm. To produce TAB tape carriers having good handling quality, the copper foil to be used in the present invention preferably has a large tensile strength, particularly a tensile strength of 400 to 700 N/mm2. To give high peeling strength after lamination between the M-surface of the copper foil and a flexible insulating film, such as a polyimide film or a polyester film, to be bonded to the M-surface, the copper foil to be used preferably has a surface roughening treatment layer formed by subjecting the M-surface to surface roughening treatment. The surface roughening treatment may be performed, for example, by carrying out cathodic electrolysis treatment using a plating bath containing copper ions and nickel ions to form a finely granular roughened surface.
The means for producing the copper foil of the present invention by forming a nickel-cobalt-molybdenum layer on a copper foil may be known one, such as electroplating, chemical plating, vacuum deposition or spattering. In view of mass productivity and good economy, practically preferred among these is cathodic electrolysis treatment using electroplating. In cases where electroplating is used, a copper foil is subjected to a cathodic electrolysis treatment in a plating bath containing nickel ions, cobalt ions and molybdic acid ions, to form a nickel-cobalt-molybdenum layer on the S-surface of the copper foil.
The plating bath to be used for the cathodic electrolysis treatment for forming the nickel-cobalt-molybdenum layer contains metal ions comprising nickel ions, cobalt ions and molybdic acid ions. Preferred examples of nickel ion sources include NiSO4, NiCl2, NiCO3 and hydrates thereof. Preferred examples of cobalt ion sources include CoSO4, CoCl2 and hydrates thereof. Preferred examples of molybdic acid ion sources include Na2MoO4, (NH4)6Mo7O24, K2MoO4 and hydrates thereof.
At least one nickel ion source, at least one cobalt ion source and at least one molybdic acid ion source are dissolved in water to prepare a plating bath. Preferred contents of the metal ions in the plating bath depend on the above-described coating amounts of the metals, current density and electrolyzing period, and are generally 0.1 to 50 g/l, preferably 5.0 to 40 g/l respectively. In cases where NiSO4xc2x76H2O as the nickel ion source, CoSO4xc2x77H2O as the cobalt ion source and Na2MoO4xc2x72H2O as the molybdic acid ion source are used, their contents are particularly preferably 2.0 to 40 g/l respectively.
The pH of the plating bath may be adjusted within a wide range including acidic values and alkaline values, and an acidic plating bath is generally preferable. The pH is preferably 4.0 to 6.0. Further, complexing agents, such as sulfamic acid, may be added to prevent the precipitation of the metal ions in the plating bath. Salts, such as sodium sulfate and ammonium chloride, may also be added to the plating bath to increase its conductivity. The temperature of the plating bath may generally be room temperature.
A nickel-cobalt-molybdenum layer is formed on the S-surface of copper foil by dipping the copper foil in the plating bath, with a non-soluble anode facing to the S-surface of the copper foil, and applying an electric current. Although the applicable current density and the electrolyzing period cannot be specified because they depend on other conditions including the above-described amounts of the metal ions and the coating amounts and ratios of the plating metals, preferred current density is generally 0.1 to 10 A/dm2, more preferably 0.2 to 2.0 A/dm2, and preferred electrolyzing period is generally 1 to 60 seconds, more preferably 1 to 30 seconds.
The nickel-cobalt-molybdenum layer may also be formed on each of the S-surface and the M-surface by subjecting the M-surface to the same cathodic electrolysis treatment.
After the copper foil is coated by the nickel-cobalt-molybdenum layer as described above, a chromate treatment layer may optionally be formed on the surface on the side of the M-surface of copper foil or each side, to improve rust resistance and chemical resistance. Further, the copper foil having the nickel-cobalt-molybdenum layer or having both the nickel-cobalt-molybdenum layer and the chromate treatment layer may further has a silane coupling agent treatment layer as an outermost layer on the surface on the side of the M-surface or each side, thereby improving bonding strength.
The copper foil for TAB tape carriers of the present invention may be preferably produced by feeding a copper foil with a desired thickness and width from a coil at a constant speed through a copper foil-treating apparatus, which consists of a series of equipments including a degreasing tank, an acid rinsing tank, a water rinsing tank, a surface roughening treatment tank and a water rinsing tank, which are optionally used, a plating tank for forming the nickel-cobalt-molybdenum layer, a water rinsing tank, an optional chromate treatment tank, an optional silane coupling agent treatment tank and a drier, and then continuously winding up.
The TAB carrier tape of the present invention is produced by applying the thus obtained copper foil for TAB tape carriers of the present invention onto a flexible insulating film, with a surface on the side of the M-surface contacting the flexible insulating film. The preferred thickness of the flexible insulating film is generally 40 to 125 xcexcm. The copper foil and the flexible insulating film may be bonded with adhesive, or by using a flexible insulating film coated with an adhesive.
The TAB tape carrier of the present invention is produced by etching the copper foil of the TAB carrier tape of the present invention to form a copper lead pattern. For example, a copper lead pattern may be produced by forming a photo resist layer of a desired pattern on the TAB carrier tape of the present invention, and carrying out etching by using the photo resist layer as a mask.
Even if the TAB tape carrier of the present invention is Sn plated on the surface of the copper leads when a semiconductor device is bonded by TAB technique, it prevents the generation of Sn whiskers and Kirkendall voids.
The present invention will be described in more detail with reference to the following Examples and Comparative Examples, which, however, do not limit the scope of the invention.